Accepted Manuscript Enhanced electrochemical performance of Li4Ti5O12 through in-situ coating 70Li2S-30P2S5 solid electrolyte for all-solid-state lithium batteries Yi Cao, Qin Li, Shuaifeng Lou, Yulin Ma, Chunyu Du, Yunzhi Gao, Geping Yin PII:
S0925-8388(18)31450-6
DOI:
10.1016/j.jallcom.2018.04.149
Reference:
JALCOM 45787
To appear in:
Journal of Alloys and Compounds
Received Date: 16 December 2017 Revised Date:
28 March 2018
Accepted Date: 12 April 2018
Please cite this article as: Y. Cao, Q. Li, S. Lou, Y. Ma, C. Du, Y. Gao, G. Yin, Enhanced electrochemical performance of Li4Ti5O12 through in-situ coating 70Li2S-30P2S5 solid electrolyte for all-solid-state lithium batteries, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.04.149. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Enhanced electrochemical performance of Li4Ti5O12 through in-situ coating 70Li2S-30P2S5 solid electrolyte for all-solid-state lithium batteries Yi Caoa,b, Qin Lia,b, Shuaifeng Loua,b, Yulin Maa,b, Chunyu Dua,b, Yunzhi Gaoa,b,*,
a
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Geping Yina,b
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion
b
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and Storage, Harbin Institute of Technology, Harbin 150001, China
Institute of Advanced Chemical Power Sources, School of Chemistry and Chemical
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Engineering, Harbin Institute of Technology, Harbin 150001, China
*Corresponding authors: Yunzhi Gao
Postal address: School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92, West Dazhi Street, Harbin 150001, China.
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Tel.: +86-451-86413721; fax: +86-451-86403807.
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E-mail addresses:
[email protected] (Y.Z. Gao)
Abstract: High energy and power densities are the urgent task for all-solid-state
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lithium batteries (ASSLIBs). Intimate contact at electrode/solid electrolyte interface and high ionic conductivity of solid electrolyte are crucial to realize high-performance all-solid-state lithium batteries. Here, we provide a liquid-phase approach to in-situ coating 70Li2S-30P2S5 (LPS) solid electrolyte onto the surface of Li4Ti5O12 (LTO) nanoparticles from mineral spirit. The cells employing LTO@LPS material in combination with the LPS solid electrolyte, vapor grown carbon fibers (VGCFs) and polybutadiene (PB) as cathode and lithium metal as anode exhibit excellent rate
ACCEPTED MANUSCRIPT capacity and cycling stability, showing reversible capacity of 110 mAh g-1 under a load for 0.5 mg cm-2 (with respect to LTO) at 1 C (1 C = 175 mA g-1) and 80℃ after 300 cycles, it even produces 118 mA h g-1 at 0.1 C under a high load for 4.25 mg cm-2.
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Keywords: in-situ coating, Li4Ti5O12, sulfide solid electrolyte, all-solid-state Lithium battery 1. Introduction
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Since the lithium ion battery was firstly commercialized by SONY Company,
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graphite, as an anode material, has been extensively used up to now. However, when it comes to PHEVs or EVs, batteries need to work at high rates and safety conditions, where the low lithium ion diffusion and lithium dendrite problems seriously limit the further applications of graphite anode. Recently, Li4Ti5O12 has been taken into
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consideration as an alternative anode[1-2] due to its excellent long cycle ability, rate ability and safety[3-4]. Nevertheless, the significant gas generation of Li4Ti5O12 anode during the charge-discharge process[5-6], which is related with the interfacial side
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reaction with liquid organic electrolyte arising from the catalysis of Ti4+ in LTO[7-9],
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leads to potential safety problems such as leakage, fire hazard, and explosion[10]. More recently, all solid-state lithium ion batteries (ASSLIBs) have attracted great
interest and been investigated more and more[11-15]. Using non-flammable inorganic solid electrolytes to substitute traditional organic liquid electrolytes, ASSLIBs present excellent safety performance. Moreover, the stable solid-state interfaces in ASSLIBs can completely solve the gas generation problem of LTO arising from the active solid/liquid interfaces. Therefore, it is urgent and important to develop solid
ACCEPTED MANUSCRIPT electrolytes with high performance and stability for LIBs. Generally, polymer-based (PEO, PPC based etc.[16-18]), sulfide (LPS, LGPS etc.[19-22]) and oxides LISICON (LLTO, LLZO etc.[23-26]) have been studied extensively as solid-state electrolytes.
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Among them, sulfide solid electrolytes, with the highest ionic conductivity of 10-3 to 10-2 S cm-1, show great potential for promising application in the commercial LIBs. Considering the high rate performance of Li4Ti5O12, combining Li4Ti5O12 anode with
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sulfide solid electrolytes into ASSLIBs seems to be a great option to achieve an
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excellent batteries system with high rate performance and superior security. However, the poor contact of active materials particles, solid electrolytes, and conductive materials is a bottleneck for developing solid-state electrolyte technology, seriously limiting the practical performance of ASSLIBs[14]. Hence, design and construction of
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rational and stable interfaces between active materials and sulfide solid electrolytes are crucial to obtain excellent electrochemistry performance. Recently, much work has been devoted to explore effective ways to build
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high-efficiency solid interfaces for ASSLIBs. Typically, pulsed laser deposition (PLD)
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technique has been successfully applied to fabricate the intimate electrode-electrolyte interface[27]. However, PLD technique is difficult to implement scale-up production. Although many other attempts such as ball milling and hot pressing technique have been widely tried, the results are limited by the physical solids contact and not desirable[28]. Instead of above-mentioned solid-phase method, liquid-phase method seems to be a more suitable way to realize the favorable interfaces. Shingo Teragawa[29] dissolved 80Li2S-20P2S5 solid electrolyte into N-methylformamide
ACCEPTED MANUSCRIPT (NMF) solution to recrystallize LPS onto LiCoO2 particles. Xiayin Yao[30] coated LPS solid electrolyte on the surface of CoS to obtain nanoscale materials with outstanding electrochemistry performance. However, the recrystallization of LPS
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solid electrolyte from solution costs its ionic conductivity. In addition, the solution (acetonitrile, di-methylbenzene, etc.) are toxic and expensive, which makes it complicated to scale up. Therefore, it is extremely in need to develop a simple and
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efficient way to construct favorable interface contact between active materials and
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sulfide solid electrolytes.
Herein, for the first time, we develop a new strategy to in-situ coat LPS solid electrolyte onto the surface of LTO nanoparticles, and a compact interphase structure is obtained. In addition, the toxic-slight mineral spirit which mainly contains heptane
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and octane was firstly employed to coat LPS electrolyte on active materials. Based on the in-situ coating method, the LTO@LPS material performs excellent rate capacity and cycling stability. This simple and efficient method for constructing intimate
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interface contact between active materials and sulfide electrolytes is easy to scale up,
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promoting the commercialization of ASSLIBs. 2. Experimental
The commercial LTO material was thoroughly mixed with Li2S (99.9%, Hawk) and
P2S5 (99%, Aladdin) in mineral spirit by constant stirring for 24 h, here we controlled the mass ratio with LTO: Li2S: P2S5: mineral spirit = 1.00: 0.07: 0.03: 1.00. Then, the mixture was dried at 60
for 24 h and subsequently annealed at 260
for 1 h in a
vacuum quartz tube to obtain the surface coated LTO material (LTO@LPS). The LPS
ACCEPTED MANUSCRIPT solid electrolyte was synthesized by similar process, solid content was controlled at 50%. X-ray diffraction (XRD) patterns of pristine, coated and annealed samples were
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acquired with an X-ray powder diffractometer (Empyrean, PANalytical) using Cu Kα1 radiation. The morphology and energy dispersive X-ray analysis (EDX) of each sample were observed by scanning electron microscope (SEM
HELIOS Nano Lab
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600i) and transmission electron microscopy(TEM, JEM-2100).
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Composite positive electrodes were prepared by mixing the LTO@LPS material (or LTO, for comparison), LPS solid electrolyte, VGCFs and PB with the weight ratio of 5: 4: 1: 0.5 in mineral spirit for 24 h, solid content was controlled at 50%. Then the slurry was casted and spread on Cu foil and dried at 60
.
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Batteries were fabricated using LPS and Li metal as the solid electrolyte and the negative electrode, respectively. The LPS solid electrolyte powder (0.20 g) was pressed into pellet in a diameter of 16 mm under a pressure of 300 M Pa. The
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above-mentioned composite electrode was pressed onto the LPS electrolyte pellet
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under 300 M Pa for 45 min. Li foil was attached on the other side of the LPS electrolyte pellet. Finally, the sandwich structure (composite electrode/LPS electrolyte/lithium metal electrode) was packaged into 2025 coin-type cells. All processes above were handled in an Ar-filled glove box (Labstar, MBRAUN). Ionic conductivity and electronic conductivity of the LPS solid electrolyte were obtained by pelletizing the powder applying a diameter of 16 mm. Stainless-steel disks were attached on both faces of the pellet. Electrochemical impedance
ACCEPTED MANUSCRIPT spectroscopy (EIS) and DC polarization measurements were performed for the cell with CS350 (Corrtest, WuHan) in the frequency range from 10 Hz to 1 MHz with an applied voltage of 10 mV. Charge/discharge measurements were conducted using a
Li/Li+) under 80
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Battery Test System (BTS-2004, Neware) in the voltage range of 2.5 ~ 1.0 V (vs. .
3. Results and Discussion
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In this work, the LPS electrolyte particles anchored on LTO nanoparticles, is
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achieved by an in-situ liquid-phase method, as illustrated in Fig. 1. The LPS electrolyte layer can effectively solve the problems of terrible physical contact between electrodes and solid-state electrolyte, serving as an intimate buffer area for ion transportation through the interfaces. At the same time, the addition of VGCFs can
materials.
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significantly improve the electron transfer efficiency from current collectors to active
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Fig. 1
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Ionic conductivity and electronic conductivity of the LPS solid electrolyte obtained by the liquid-phase approach were characterized by EIS and DC polarization measurements (see Fig. S1). Ionic conductivity and electronic conductivity of the LPS solid electrolyte were measured to be 2.67 × 10 S/cm and 1.91 × 10 S/cm, respectively. It conforms to the data reported before[35], which suggests it is feasible to prepare LPS soild electrolyte via liquid-phase approach from mineral spirit. And Fig. 2a shows XRD patterns of the LPS solid electrolyte, LTO@LPS materials, and the spinel LTO standard pattern (JCPDS #49-0207) with an Fd3m space group.
ACCEPTED MANUSCRIPT Obviously, the spinel bulk structure of LTO did not change in LTO@LPS material. However, owing to the strong diffraction peaks of LTO and low contents of LPS electrolyte, diffraction peaks of the LPS solid electrolyte were hardly detected at the
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diffraction pattern of LTO@LPS material. From Fig. 2b, SEM shows that the LTO@LPS particles retain the original morphology of LTO particles, and the obtained
which is similar to the original LTO nanoparticles.
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composite material presents homogeneous distribution in the size of about 500 nm,
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To confirm the composition and elements dispersion of LPS solid electrolyte on the surface of LTO particles, EDX mapping (Ti, P and S) test of LTO@LPS nanocomposite was performed. Fig. 2c shows that the mapping areas of P and S were overlapped the area of Ti, indicating the LPS solid electrolyte was uniformly
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distributed throughout the LTO nanoparticles. Fig. 2d shows the typical SEM image of the composite electrode. It can be clearly seen that the LTO@LPS material, LPS solid electrolyte, and VGCFs are uniformly distributed in the three-dimensional scale.
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The construction of successive electron and lithium-ion pathways will effectively
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contribute to a good electrochemical performance of the ASSLIBs.
Fig. 2
Coating thickness was characterized by TEM measurement. According to Fig. 3a
and 3b, the coating thickness of the LPS layer (presented as an amorphous layer) on the LTO particles is around 2nm. Considering the ultrasonic dispersion during sample preparation and sulfide electrolytes’ inevitable reaction with moisture, the coating
ACCEPTED MANUSCRIPT thickness should thicker than 2nm. In contrast, Fig. 3c and 3d shows the microstructure of the composite electrode using LTO material, obviously, there is no amorphous layer on the LTO material surface. Moreover, the TEM results directly
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proves that our method constructs an intimate electrode/electrolyte interface. In addition, we have calculated the interfacial charge resistance of LTO@LPS and LTO materials to be 461 Ω and 4275 Ω respectively (see Fig. S2), which further reflects
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our in-situ coating method dramatically enhances the electrode/electrolyte interface
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contacts.
Fig.3
In order to prove our conjecture, electrochemical performance of the LTO@LPS
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and LTO materials were evaluated. Fig. 4a presents the charge-discharge profiles (0.05 C) of the LTO@LPS and LTO particles in the voltage range of 1.0 - 2.5 V at 80
. The voltage plateau observed in the charge-discharge curves is consistent with
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the typical LTO plateaux 1.55 V. However, the LTO@LPS composite electrode
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delivers an initial specific discharge capacity of 198 mA h g-1 and a reversible charge capacity of 168 mA h g-1 with a columbic efficiency of 84.8%. In contrast, the LTO composite electrode delivers an initial specific discharge capacity of 31 mA h g-1 and a reversible charge capacity of 22 mA h g-1 with a columbic efficiency of 70.9%. The low columbic efficiency in the 1st cycle was caused by the side reaction between the LPS electrolyte and Li anode. At the same time, the over potential in the charge-discharge curves of LTO@LPS and LTO materials was 0.03 V and 0.05 V,
ACCEPTED MANUSCRIPT respectively (see Fig. S4). It confirms that the improved contacts between LTO nanoparticles and LPS solid electrolyte effectively lead to elongated charge/discharge plateaus and smaller polarization, indicating enhanced electrochemical kinetics and
the LTO@LPS and LTO materials at 0.05 C under 80
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surface electrochemical reactivity[31-33]. The corresponding cyclic performance of was shown in the Fig. 4b.
Commonly, a slow rate condition requires much severe interface structure. Under
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such a slow rate of 0.05 C, LTO@LPS material still presents good capacity retention,
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which further confirms the in-situ coating method constructs an intimate interface between LTO particles and LPS solid electrolytes.
Fig.4
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To understand the enhanced electrochemical kinetics, a long-term cycling stability at 1 C were investigated, as shown in Fig. 5a and 5b. The LTO@LPS material delivers a stable capacity of 110 mA h g-1 after the 300 cycles, relatively, the pure LTO
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material shows the capacity of only 2 mA h g-1 (see Fig. S3). Interestingly, there is a
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remarkable capacity loss at the initial 10 cycles during the cycling, the lowest capacity presents to be 87.5 mA h g-1. However, the subsequent 50 cycles show the capacity regains and tend to stabilize for the rest of cycles. A discharge capacity of 110 mA h g-1 with a capacity retention of ~61% after 100 cycles is obtained. This phenomenon can be explained as follows. Generally, the electrochemical stable window (ESW) of the LPS solid electrolyte has been reported to be 1.7 ~ 2.1 V (vs. Li/Li+)[34], and herein the cells were operated in the voltage of 1.0 ~ 2.5 V (vs. Li/Li+), which is out
ACCEPTED MANUSCRIPT of the range of the ESW of the LPS solid electrolyte. The initial capacity loss (in 10 cycles) was dominated by the decomposition of electrolyte on the LPS solid electrolyte and Li metal anode interfaces. The majority of Li2S decomposed products
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generates a interfacial layer with huge resistance, causing a significant polarization and capacity loss ( Fig. 5b). However, the subsequent cycles were dominated by an unexpected oxidation reaction of the LPS solid electrolyte. To be specific, when cells
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were charged over 2.1 V, the LPS solid electrolyte decomposes and generates S,
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which can act as an active material and supply an extra capacity. As has been demonstrated, the redox reaction of S operated in the window of 2.0 ~ 2.5 V[35], where a typically 2.3 V plateau was observed at Fig. 5b. After 100 cycles, the side reaction fades away and the stable discharge capacity appears to be 110 mA h g-1.
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Columbic efficiency of the cell is also coherent to this phenomenon, the columbic efficiency quickly enhanced from 64.3% to 97.3% in the initial 10 cycles, and then eases up to 99.5% in the subsequent 50 cycles. At last, the efficiency is close to 100%
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and stable in the subsequent cycles. Moreover, the oxidation peak at 2.35 V in Fig. 5c
V.
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further confirms that LPS solid electrolyte is not stable when it was operated over 2.1
Fig.5
In order to study the practical application prospects of the LTO@LPS material, rate performance (Fig. 6a) and the relationship between activate materials loading and specific capacity (Fig. 6b) were investigated. Although both materials show stable
ACCEPTED MANUSCRIPT capacities at various rates, the LTO@LPS material exhibits much higher rate capacity with the increasing of current densities than the LTO material. When the rate reduced to 0.05 C again, about 97.6% of the initial capacity of the LTO@LPS material was
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recovered. Fig. 6b shows the relationship between the LTO@LPS material loading and specific capacity of the LTO@LPS type cells at 0.1 C and 80
, and it can be
seen that the capacity of the cells decrease slowly with the increase of the LTO@LPS
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material loading. Above all, we find that the capacity of the cells even achieve 118
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mA h g-1 under a high LTO loading of 4.25 mg cm-2, higher than all the performance reported before[36-40]. However, the capacity sharply decreased to 48 mA h g-1 when the loading increases to 12.5 mg cm-2, which suggests that high loading of LTO simultaneously enlarges the Li+ conductive pathway, and therefore makes a difficult
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diffusion of Li+ in the cathode away from the electrode/electrolyte interface.
4. Conclusion
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Fig.6
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In summary, the LTO coated LPS solid electrolyte material was successfully prepared from mineral spirit for the first time, and the LTO@LPS material showed much higher charge-discharge capacity and excellent rate performance than the pure LTO material. It can be concluded that the intimate electrode/electrolyte interfaces were formed by in-situ coating LPS onto the surface of LTO particles. In addition, the PB plus mineral sprit system in this work makes ASSLIBs possible to be fabricated by traditional lithium ion batteries equipment. Acknowledgements
ACCEPTED MANUSCRIPT This work was financially supported by the National Natural Science Foundation of China (Grant No. 21373072 and No.51202047). References
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Figure captions
Fig. 1 Scheme of synthesis of LTO@LPS nanoparticles and LPS solid electrolyte. Fig. 2 (a) XRD Patterns of the standard LTO JCPDS card and LTO@LPS 10 wt%
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nanocomposites; (b) the SEM and (c) EDX mapping (Ti, P and S) images of
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Fig. 3 (a) TEM image and (b) its partial enlargement of the LTO@LPS material; (c) TEM image and (d) its partial enlargement of the composite electrode using LTO material Fig. 4 (a) charge-discharge profiles and (b) cycle performance at 0.05 C of the LTO@LPS and LTO materials operated at 80
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ACCEPTED MANUSCRIPT Fig. 5 (a)The Cycling stability (b) charge–discharge curves of different cycles at 1C under 80
‐and (c) cyclic voltammetry of the LTO@LPS material at 0.05 mV/s.
Fig. 6 (a) the rate performance of the LTO@LPS and LTO materials operated at 80
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(b) the relationship between LTO loading and specific capacity of the LTO@LPS
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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Fig. 6
ACCEPTED MANUSCRIPT Highlights LTO@LPS material is prepared through an in situ liquid phase method.
In situ coated LPS electrolyte constructs an intimate interface contact with the LTO material.
The LTO@LPS material produces 118 mA h g-1 at 0.1 C under a high load for 4.25 mg cm-2.
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